J. Chil. Chem. Soc., 50, N° 3 (2005), págs: 581-585

SYNTHESIS AND CHARACTERIZATION OF FUNCTIONALIZED VINYL COPOLYMERS I. STRUCTURE-MONOMER REACTIVITY RELATIONSHIP IN COPOLYMERS CONTAINING N-VINYL-2-PYRROLIDONE MOIETIES

NICOLÁS GATICA^1*, NATALÍ FERNÁNDEZ¹, ALEJANDRA OPAZO², DEODATO RADIC'³

¹Departamento de Polímeros, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile.
²Dirección de Ciencias Básicas, Departamento de Química, Universidad Iberoamericana de Ciencias y Tecnología, Santiago, Chile.
³Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile.
E-mail address: nigatica@udec.cl, fax: 56-41-245974.

SUMMARY

Copolymers containing N-vinyl-2-pyrrolidone as a common monomer with acrylic acid, 4-methyl styrene and vinyltrichlorosilane of different comonomer compositions were synthesized and characterized. Copolymer composition was determined by elemental analysis, from which monomer reactivity ratios (MRR, r) were estimated using straight line intersection procedures, such as Fineman-Ross and Kelen-Tüdõs methods, and a nonlinear one, the Reactivity Ratios Error in Variables Model. In this case, the values of MRR are r_VP = 0.1 and r_AA = 1.1 (for VP-co-AA), r_VP = 0.8 and r_4MS = 14.6 (for VP-co-4MS), and r_VP = 0.0 and r_VTCS = 0.3 (for VP-co-VTCS). Copolymers containing blocks of one of the monomer units and another tending to the alternation were obtained, with the product depending on the chemical nature of the comonomers. The different proposed comonomer distributions are analyzed in terms of the obtained MRR values and compared with related systems. The MRR are discussed in terms of monomer structural properties such as electronegativity and electron delocalization.

Keywords: functionalized vinyl copolymers; monomer reactivity ratios; comonomer sequence and distribution; N-vinyl-2-pyrrolidone.

The chemical structure of a copolymer depends not only on the two monomer units forming the macromolecule, but also on how such units are distributed along macromolecular chains [1-3]. This distribution is a direct consequence of each monomer's reactivity in the copolymer molecule [4-7]. In radical copolymerization, the reactivity of a free radical depends on the nature of the side groups linked to the radical carbon. With respect to this structural feature, there are three important factors influencing the monomer reactivity in radical copolymerization of vinyl monomers: electronic delocalization, polarity and the volume of the side group. All these factors can be studied by analyzing the reaction between functionalized vinyl monomers to obtain the corresponding functionalized vinyl copolymers, which are understood as macromolecular compounds formed by monomer units containing groups of different chemical nature.

The aim of this work is the synthesis, characterization and estimation of the monomer reactivity ratios (MRR, r) of copolymers of VP with acrylic acid (AA) [poly(N-vinyl-2-pyrrolidone-co-acrylic acid) (VP-co-AA)], with 4-methyl styrene (4MS) [poly(N-vinyl-2-pyrrolidone-co-4-methyl styrene) (VP-co-4MS)], and with vinyltrichlorosilane (VTCS) [poly(N-vinyl-2-pyrrolidone-co-vinyltrichlorosilane) (VP-co-VTCS)] (Scheme 1). In this way, it would be possible to check the reactivity of a monomer derived from vinylsilane as well as monomers containing carbonyl and aromatic groups in the side chain. The effects of the chemical structure on comonomer distribution in the macromolecular chains can also be analyzed.

EXPERIMENTAL

Monomer and copolymer preparation

Commercial samples of VP, AA, MS and VTCS from Aldrich^® were distilled under vacuum before copolymer synthesis. Copolymers were obtained by radical polymerization in bulk at 323 K under nitrogen and a,a'-azobisisobutyronitrile (AIBN) as initiator. The monomer feed ratio was varied in a series of copolymerizations of both monomers. Copolymerization time was controlled to obtain low monomer-to-copolymer conversions. The copolymers were purified by dissolution in methanol (VP-co-AA) and 1,4-dioxane (VP-co-4MS, VP-co-VTCS) and precipitation with benzine/ethyl ether mixture (VP-co-tBA) and methanol (VP-co-4MS, VP-co-VTCS). Copolymer samples were dried in vacuum at 298 K until constant weight.

Copolymer characterization

The copolymers were characterized by ¹H-NMR in a Bruker AC 250P spectrometer using tetramethylsilane (TMS) as an internal standard. FTIR spectra in KBr were recorded using a Nicolet Magna-IR 550 instrument. Viscosity measurements were carried out with an Ostwald viscometer with negligible kinetic energy corrections. Intrinsic viscosity ([h]) was determined according to the Solomon-Gotessman relationship [13]. Copolymer compositions were determined by elementary analysis, following the variation of nitrogen content arising from the VP comonomer units.

RESULTS AND DISCUSSION

Intrinsic viscosities ([h]) in appropriate solvents at 298 K for unfractionated samples of VP-co-AA, VP-co-4MS and VP-co-VTCS are summarized in Table 1. [h] values should be useful in estimating qualitatively the degree of polymerization. ¹H-NMR and FTIR spectra are in agreement with the expected copolymer structure. When the monomer and copolymer spectra are compared in the ¹H-NMR study, the signal corresponding to the vinyl protons disappears from the spectral region close to 6.0 ppm. The comparative study of the FTIR spectra of the copolymers with regard to the corresponding monomers allows confirmation of the ¹H-NMR results. The following characteristic absorption bands (in cm^-1) are observed: 3449 (carboxylic acid O-H stretching), 2961 (alkane C-H stretching), 1731 (carboxylic acid carbonyl stretching), 1639 (tertiary amide carbonyl stretching), 1447 (alkane ­CH₂­ bending), 1366 (alkane C-H bending) [for VP-co-AA]; 2921 (alkane C-H stretching), 1689 (tertiary amide carbonyl stretching), 1632 (aromatic C-C stretching), 1510 (alkane ­CH₂­ bending), 1442 (alkane CH₃ bending), 1374 (alkane C-H bending), 812 (p-substituted aromatic blending) [for VP-co-4MS] and 2967 (alkane C-H stretching), 1654 (tertiary amide carbonyl stretching), 1460 (alkane ­CH₂­ bending), 1420 (alkane C-H bending), 595/547 (Si-Cl stretching) [for VP-co-VTCS]. The solubility of three different compositions of each copolymer was checked in a variety of solvents, and the copolymers are soluble in a wide variety of them.

Table 1. Intrinsic viscosity ([h]) and feed (fVP) and copolymer (FVP) compositions for the copolymers studied.

(a) Solutions with the proper concentration for applying the Solomon-Gotessman relationship (0.18 g/dL) [13] could not be obtained. (b) At 298 K in methanol for VP-co-AA and 1,4-dioxane for VP-co-4MS and VP-co-VTCS.

Compositions of the feed (f) and the resulting copolymers (F) are also compiled in Table 1. One of the first experimental evidences of a determined monomer's reactivity is the obtained copolymer composition. Figure 1 shows the feed composition (f) v/s copolymer composition (F) diagrams for the studied copolymers (the discontinuous line represents the case of an ideal copolymerization, i. e., f = F). In the case of VP-co-4MS (Figure 1-b), there is low incorporation of the VP monomer. This result can be interpreted in terms of the effect of the aromatic group of 4MS on its corresponding growing radical in the propagation step. This group induces electronic delocalization over these radicals, a factor contributing to the stabilization. Thus, high incorporation of 4MS in the copolymer is obtained, meaning low incorporation of VP. When VP is copolymerized with AA (Figure 1-a), a similar behaviour is observed. In this case, the positive charge density generated on the carbonyl carbon atom can favour a significant electron attraction in the AA radicals. When VP is copolymerized with VTCS (Figure 1-c), there is a higher incorporation of VP into the macromolecule. In this copolymer, the comonomer of VP lacks functional groups contiguous to the macromolecular backbone, and there is a higher VP content in the copolymer. However, it seems clear that the presence of three electron-attracting chlorine atoms for each VTCS monomer unit gives rise to a significant attraction of the free electron generated in the growing macromolecular chain and the stabilization of the corresponding macroradical. For this reason, an incorporation of VTCS into the copolymer chain of the same order of VP is obtained for VP-co-VTCS (Figure 1-c). In fact, it is possible to observe an azeotropic copolymer composition at about 40 mol % in VP (point P), a characteristic behaviour of this system type [6].

Figure 1. Copolymer composition (FVP) variation with feed composition (fVP) for: a) VP-co-AA, b) VP-co-4MS and c) VP-co-VTCS.

In order to study the monomer reactivity and to characterize the comonomer distribution, the monomer reactivity ratios (MRR) r₁ and r₂ were determined following previously reported mathematical procedures. The least squares methods, according to Fineman and Ross [14] (FR) and Kelen and Tüdõs [15] (KT), were used, plotting G against F and h against x, respectively (equations 1 and 2).

G = F r₁ ­ r₂ (1)

h = (r₁ + r₂/ a) x ­ r₂/a (2)

with G = x(y ­1)/y, F = x²/y, h = G/(a + F) and x = F/(a + F),

where: a = (F_l F_h)^1/2 [F_l and F_h are the lowest and highest values of F, respectively]

[f_i and F_i are the feed and resulting compositions for the i-monomer in the copolymer]

Table 2 presents the copolymerization parameters according to the FR and KT methods at different compositions. Data consider VP as monomer 1. From these values, the corresponding FR and KT plots were obtained. Figure 2 shows both representations for VP-co-AA, i. e., G v/s F and h v/s x, as a representative example.

Table 2. Copolymerization data for VP-co-AA, VP-co-4MS and VP-co-VTCS: composition ratios x and y, Fineman-Ross11 parameters G and F and Kelen-Tüdõs12 parameters h, x and a

Figure 2. Fineman-Ross (a) and Kelen-Tüdõs (b) plots for VP-co-AA.

The MRR of VP-co-AA, VP-co-4MS, and VP-co-VTCS were also calculated using a computer program based on a statistically valid non-linear minimization algorithm: Reactivity Ratios Error in Variable Method (RREVM) [16], using as starting values the MRR obtained by the KT method, which gives more reliable values than the FR procedure due to a better distribution of the points along the h-x axis as Figure 2 demonstrates. Figure 3 shows the 95 % probability contour for MRR of VP-co-AA given by the RREVM method. Similar contours are obtained for different running conditions of the program for both VP-co-AA, for VP-co-4MS and VP-co-VTCS. In all cases, the MRR values obtained were generated using errors of 2 % for the monomer feed compositions and 5 % for the copolymer compositions.

Figure 3. 95 % posterior contour for estimated MRR of VP-co-AA.

The MRR obtained by the least squares methods (FR and KT) and by the RREVM procedure are compiled in Table 3, where a good agreement between the different sets of data is observed. The tendency is always the same in both the linear methods and the RREVM one.

Table 3. Monomer reactivity ratios obtained by FR, KT and RREVM methods for VP-co-AA, VP-co-4MS and VP-co-VTCS.

Using the RREVM values as a reference, it is possible to note that AA shows a trend to form blocks (r_AA = 1.1) in VP-co-AA. An earlier work of van Paesschen and Smets [17] reports values of r_AA = 1.3 and r_VP = 0.15 for this copolymer synthesized in bulk at 75 C. Other authors describe the monomer reactivity in this system synthesized in organic [18] and aqueous [19] solutions. In these works, only the FR method is used. Nevertheless, in all the cases r_AA > r_VP and in most them r_AA > 1, and r_VP tends to zero. The electronic stabilization effect exerted by the carbonyl group of the ester adjoined to the carbon carrying the radical electron on the AA radical in the propagation step explains the higher reactivity of AA in this copolymerization. Dinçer et al. [20] and Pekel et al. [21] had previously reported this behaviour for similar copolymer systems containing N-isopropylacrylamide and ethyl methacrylate, respectively. The situation is similar for VP-co-4MS, which contains one monomer with a monomer reactivity ratio higher than the unity (r_4MS = 14.6) and another with a value lower than the unity (r_VP = 0.3). The high value obtained for 4MS can be interpreted in terms of the stabilization by resonance of the 4MS radicals discussed above. Gatica et al. [4,22] have described this behaviour for other functionalized copolymers containing aromatic units of 2-vinylpyridine and 4-vinylpyridine.

When two monomers have MRR equals to zero, the corresponding copolymer shows an alternating comonomer distribution [23]. Each monomer suffers a cross propagation (i. e., each time the monomer incorporates into the macromolecule, it will react with a molecule of the other monomer). The behaviour observed in VP-co-VTCS (r_VP = 0.0 and r_VTCS = 0.3) permits its classification as a copolymer with a high tendency to alternation. Additionally, the MRR values for VP and VTCS in this copolymer show the effect of the chlorine atoms on the VTCS reactivity. This one is of the same order of VP, in spite of the fact that the copolymerization of vinyl silane-derived monomers is generally a difficult process [9,22,24].

CONCLUSIONS

MRR values were obtained for the studied copolymers using both linear methods and the RREVM non-linear procedure. A good agreement is observed between the different methods. The RREVM non-linear procedure estimated the following values: r_VP = 0.1 and r_AA = 1.1 (for VP-co-AA), r_VP = 0.8 and r_4MS = 14.6 (for VP-co-4MS), and r_VP = 0.0 and r_VTCS = 0.3 (for VP-co-VTCS).

The particular MRR values seem to indicate the presence of zones with a high concentration of 4MS units in VP-co-4MS, which was interpreted in terms of 4MS block formation. This copolymer contains some units of VP between blocks of 4MS. Although to a lesser extent, a similar situation is observed for VP-co-AA. In the case of VP-co-VTCS, a tendency to alternation is observed. Thus, the influence of the vinyl monomer's side-chain nature on its reactivity and on comonomer distribution was revealed.

ACKNOWLEDGEMENTS

We express our thanks to Dirección de Investigación Universidad de Concepción, Grant 201.025.022-1.0 for financial support.

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